I got fined, showed them the EMS receipt, even had a copy of the paper they signed to say they received it, they lost it somewhere in their system, cost me 2000 baht fine, and a notation in my passport, so now do not trust them by mail in....
I have heard that too, although it has not happened to me, last time I renewed my extension of stay, they gave me a new 90 day date, so will be interesting to see how that goes next time I try to do it on line